The severe service conditions typical of internal combustion engines, e.g., diesel engines, generally require the pistons in such engines to be formed of a strong and durable material. To address these needs, the pistons used in diesel engines are often made of steel. Additionally, the extremely hot combustion chamber temperatures often require the pistons be cooled during engine operation. For example, a piston crown in a diesel engine may be cooled with an oil spray directed at the bottom of the crown and with an internal feature of the crown such as an oil gallery. The oil spray and oil circulating in the gallery may remove some of the excessive heat generated during engine operation.
To facilitate the construction of a piston assembly with an oil gallery in the piston crown, a two part piston is used in some diesel engines. The top piece, the crown, is the part of the piston exposed to the extremely hot combustion temperatures. The bottom piece, called the skirt, generally provides piston guidance within the cylinder. The crown or skirt may also provide a holder for the piston rings that interface with cylinder bore surfaces, such as ring grooves that receive the piston rings.
Current methods of joining a piston crown and skirt have inherent limitations. Vacuum brazing, for example, is a relatively expensive and time consuming process. Vacuum brazing requires the work piece be processed in a controlled environment.
Proper alignment of features on the crown and skirt is critical to proper engine performance. Tolerances for the finished piston feature alignment are typically in the tenths of millimeters, therefore, the tolerance for error in alignment of the crown and skirt in the blank is also extremely small. Thus, dynamic processes, such as friction welding, may be difficult to execute while meeting the exacting tolerances needed of the finished piston assembly. More specifically, friction welding typically involves spinning one of the two piston components being joined. Stopping the crown or skirt in the exact spot to correctly orient it with the other piece is generally difficult to accomplish.
Additionally, inconsistent weld thicknesses can also cause problems with the piston during operation. A weld that is too thick or too thin will cause the vertical or Z-dimension of the piston to be out of tolerance. Out of tolerance pistons may prematurely fail in service, or cause damage to engine components. The friction welding process does not add metal to the system to form the metallurgical bond between the two component pieces. Rather, metal from the pieces is sacrificed to from the bond between the two parts. This may cause the Z dimension of the two joined parts to change inconsistently, and may cause scrap, or premature failure of the engine components.
Some pistons have intricate, internal features that cannot be reached with standard welding techniques, and offset or asymmetrical features may not be easily formed in welding techniques. The clearances and access to these features are usually small and standard welding equipment cannot be used. The geometry and alignment requirements of the features internal to the crown eliminate friction welding as a feasible option to join piston components with intricate internal features.
Accordingly, there is a need for an improved piston assembly and method of making the same that addresses the above problems.